The present invention relates to alumina aerogels, particularly to a method for producing strong, low density, high porosity, alumina aerogel monoliths, and more particularly to a two-step sol-gel method of preparing high porosity, pure, alumina aerogel by combining the use of substoichiometric water for hydrolysis, the use of acetic acid to control hydrolysis/condensation, and high temperature supercritical drying.
Alumina-based ceramics, in addition to their relatively high strength, are noted for their enhanced thermal and chemical stability. These properties of alumina have been shown to also apply to low density aerogel materials. To date, the major use for alumina-based aerogels is as high temperature, high surface area catalyst supports. Monolithic alumina aerogels, which have been difficult to produce by current synthetic procedures, would be capable of providing thermal insulation over a larger temperature range than the more common silica aerogels and would be a useful material for many applications, such as for alumina aerogel crucibles of use in improved high temperature alloy melt processing.
It is very difficult to manufacture alumina (Al2O3) aerogels in monolithic form, since alumina aerogels usually react with moisture in air and ultimately fall apart.
The sol-gel synthesis of alumina gels has been well studied and it has been found that the complex solution chemistry of aluminum hydroxide complicates the structural formation of the gel. It has been shown that there are several possible structural variations in the aluminum alkoxide derived gels. Some of the variables that affect the final structure of the gel include: the type of alkoxide used, the ratio of alkoxide to water, the rate of hydrolysis, the temperature of drying, the type of catalyst used, the pH of the solution, and the temperature at which the reactions occur. The final form of the aerogel may be monolithic or powder, amorphous or crystalline, or biphasic, heterogenous or homogenous.
Considerable work has been carried out and papers published for both pure alumina aerogels and composite alumina-silica aerogels. See UCRL-JC-137749, J. F. Poco et al, Synthesis of High Porosity, Monolithic Alumina Aerogels, Oct. 9, 2000. The synthesis of monolithic, stable, high porosity alumina aerogels has been found to be especially difficult due to the complex chemical pathways leading to gelation, the susceptability to cracking during drying, and the hygroscopic nature of the dried material. Only few papers describe the formation of truly monolithic aerogels containing alumina, and the highest porosity, pure alumina aerogel reported was xcx9c95% porous (see Y. Mizushima et al, J. Non-Crystalline Solids 167 [1994]1).
There is a need for very high porosity ( greater than 95%) monolithic alumina aerogels for space applications. Thermal protection is needed for space vehicles and thermal insulation is required for many of the experiment assemblies. Monolithic alumina aerogels could be used separately or they could be added to other thermal protection materials to make composites with superior thermal resistance properties. For space applications, lightweight is the most important requirement, thermal resistance is next, and finally robustness and stability.
The method of the present invention produces monolithic aerogels that exhibit all of the above-referenced features. The monolithic gels or the present invention are made in two steps: first, an alumina sol is made, and second, the sol is gelled. The alumina sol is made following a variation of the Yoldas procedure (see B. E. Yoldas, Am. Ceramic Soc. Bull., Vol. 54, No. 3 [1975] 286), using less than the stoichiometric amount of water instead of a large water excess, as in Yoldas. The method of this invention is similar to the method reported by Himmel et al, J. Non-Crystalline Solids 186 (1995) 149, with some variations in the chemistry and the drying.
It is an object of the present invention to provide high porosity ( greater than 95%), monolithic, alumina aerogels.
A further object of the invention is to provide a method for producing high quality alumina aerogels.
A further object of the invention is to provide a method for producing alumina aerogels which are superior in their ability to withstand temperature of 1000xc2x0 C. and not shrink.
Another object of the invention is to provide a method for producing monolithic, high porosity, alumina aerogels with a change in the linear dimensions (shrinkage) of less than 2%.
Another object of the invention is to provide a method for producing strong, low density alumina aerogel monoliths using a two-step sol-gel process which combines the use of substoichiometric water for hydrolysis, the use of acetic acid to control hydrolysis/condensation, and high temperature supercritical drying.
Another object of the invention is to provide a two-step method for producing strong, low density, high porosity alumina aerogel monoliths which have a polycrystalline aerogel microstructure.
Another object of the invention is to provide a method for producing alumina monoliths having low thermal conductivity up to 800xc2x0 C. and do not undergo structural changes up to 1050xc2x0 C.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawing. The present invention involves the synthesis of high porosity, monolithic, alumina aerogels. The method is a two-step sol-gel operation wherein pure alumina aerogels are produced having a porosity of greater than 95%. The alumina aerogels thus produced have a polycrystalline morphology and have physical properties superior to those of silica aerogels for equivalent densities, and include greater mechanical strength, lower density/greater porosity, enhanced thermal resistance, lower thermal conductivity up to 800xc2x0 C., and resistance to structural changes up to 1050xc2x0 C. Applications for the alumina aerogels produced by the method of this invention include high surface area catalyst supports capable of high temperature, thermal insulation over a wider range than possible with silica aerogels, and space applications-light-weight thermal insulation. The monolithic alumina gels are made in two-steps: first an alumina sol is made, then the sol is gelled in the second step. The alumina sol is made following a variation of the Yoldas procedure, referenced above, using less than the stoichiometric amount of water instead of a large water excess, the use of acetic acid, and high temperature supercritical drying, which contribute to the formation of a polycrystalline aerogel microstructure.